1. Field of Invention
This invention relates to a thin film formation method by an ink jet method, an ink jet apparatus, a production method of an organic EL (electro-luminescence) device, and an organic EL device.
2. Description of Related Art
An organic EL display including organic EL devices (light emitting device having a light emitting layer made of an organic material interposed between an anode and a cathode) so arranged as to correspond to pixels has been rapidly developed in recent years as a spontaneous light emitting display that will replace current liquid crystal displays. Materials of the light emitting layer of the organic EL device can include aluminum quinolynol complexes (Alq3) and poly(paraphenylene)vinylene (PPV) as an organic material having a high molecular weight.
As disclosed in xe2x80x9cAppl. Phys. Lett.xe2x80x9d 51(12), Sep. 21, 1987, 913, it is known to form a film of a light emitting layer made of an organic material having a low molecular weight by vacuum evaporation. Another reference, xe2x80x9cAppl. Phys. Lett.xe2x80x9d 71(1), Jul. 7, 1997, p.34 et seq. describes the film formation of a light emitting layer made of an organic material having a high molecular weight.
In organic EL devices for display, it is necessary to form an anode at each pixel position on a substrate and to dispose a light emitting layer on each anode. If the arrangement of the light emitting layer can be performed by an ink jet method, precise patterning can be made within a short time because application and patterning can be conducted simultaneously. Moreover, since the amount of materials to be used is that which is minimum necessary, the materials can be used without waste and the production cost can be lowered.
To arrange the light emitting layer by an ink jet method, it is necessary to use a material in the liquid form. When a polymer material such as PPV is used as the material of the light emitting layer, the arrangement can be made by the ink jet method if a precursor solution of the polymer material is used. Japanese Patent Laid-Open Publication Nos. 11-40358, 11-54270 and 11-339957 teach to arrange a light emitting layer made of a PPV type polymer material in accordance with the ink jet method.
As shown in FIG. 1A, in the liquid arrangement by the conventional ink jet method, an ink jet head 2 smaller than a substrate 1, for example, is employed. The inside of the surface of the substrate 1 is so divided into a plurality of regions 11 to 15 as to correspond to the length of rows of nozzles 3 of the head 2. The liquid is serially discharged from the nozzles 3 of the head 2 while the substrate 1 or the head 2 is being moved.
According to this method, however, when a solvent of the liquid to be discharged is a solvent having a large density, a solvent vapor evaporating from droplets is likely to stay inside the substrate surface. When a droplet A having an early arrangement order on the substrate is compared with a droplet B having a late arrangement order, for example, as shown in FIG. 1B, the droplet B having a late arrangement order is discharged in an atmosphere in which the partial pressure of the solvent vapor is high. As a result, a drying rate of the droplet B is lower than that of the droplet A. The droplet A arranged previously, too, is affected by the solvent vapor staying inside the substrate surface and in some cases, it is again dissolved after drying or its drying rate becomes lower.
Therefore, when a solution prepared by dissolving a plurality of polymer materials having mutually different molecular weight or polarity in a solvent having a large density is arranged on the substrate by the conventional ink jet method, the droplets having a low drying rate are likely to result in a thin film in which a plurality of polymer materials are in the phase separation state. When the drying rates of the droplets are different inside the substrate surface, the condition of the resultant thin film becomes different depending on the position inside the substrate surface.
As described above, when the conventional ink jet method is employed to arrange the light emitting layer in the organic EL display, luminance is likely to vary inside and among pixels.
In view of the problems with the conventional technologies, the present invention is directed to obtain a thin film having high uniformity inside a substrate surface even when a solvent of a liquid to be discharged has a large density in a formation method of a thin film by an ink jet method.
In a method of forming a thin film by an ink jet method including the step of discharging a liquid containing thin film-forming materials and a solvent from liquid discharge ports to each position on a substrate while the liquid discharge ports are being moved relatively to the substrate, the present invention provides a thin film formation method by an ink jet method characterized in that subsequent droplets are arranged while a solvent vapor evaporating from droplets arranged previously on the substrate is compulsively removed from inside the substrate surface.
In a method of forming a thin film by an ink jet method including the step of discharging a liquid containing thin film-forming materials and a solvent from liquid discharge ports to each position on a substrate while the liquid discharge ports are being moved relatively to the substrate, the present invention provides a thin film formation method by an ink jet method characterized in that a solvent vapor evaporating from droplets arranged previously on the substrate is compulsively removed from inside the substrate surface immediately after the arrangement of the droplets.
The present invention further provides an ink jet apparatus including gas blowing means for blowing a gas to a surface of a liquid discharged surface on which droplets have already been arranged.
The present invention provides a thin film formation method by an ink jet method including the steps of moving relatively liquid discharge ports with respect to a substrate, discharging a liquid containing thin film-forming materials and a solvent to each position of the substrate from the liquid discharge ports and arranging successively droplets at positions on the substrate. The present invention further has a feature in that a solvent vapor evaporating from the droplets arranged previously is compulsively removed from inside the substrate and subsequent droplets are arranged.
Accordingly, even when the solvent of the liquid to be discharged has a large density, the method described above prevents the solvent vapor evaporating from the droplets arranged previously on the substrate from staying inside the substrate surface. Consequently, the liquid at positions of a late arrangement order can be discharged at a low partial vapor pressure of the solvent. A solvent having a large density is, for example, cyclohexylbenzene, tetralin, tetramethylbenzene, dodecylbenzene or diethylbenzene.
In this way, it becomes possible to prevent a drying rate of the droplets at the positions of the late arrangement order from becoming lower than that of the droplets at positions of the early arrangement order. It also becomes possible to prevent the droplets at positions of the early arrangement order from being re-melt after drying, and to prevent the drying rate of such droplets from being retarded. Since the delay of the drying rate of the droplets can thus be prevented, formation of a thin film, in which a plurality of polymer materials having different molecular weight and different polarity exist in a phase separation state, can be prevented even when a solution in which such polymer materials are dissolved in a solvent having a great density is used as a discharging liquid.
In a thin film formation method by an ink jet method including the steps of discharging a solution containing the thin film-forming materials and the solvent from liquid discharge ports to each position of a substrate while the liquid discharge ports are being moved relatively to the substrate, and thus arranging serially a droplet at each position of the substrate, the present invention has another feature in that a solvent vapor evaporating from the droplets arranged on the substrate are compulsively removed from inside the substrate surface immediately after the arrangement of the droplets.
According to this method, the solvent vapor evaporating from the droplet arranged on the substrate rapidly becomes absent inside the substrate surface. Therefore, the droplet previously arranged is prevented from being affected by the solvent vapor evaporating from the droplet arranged subsequently. Thus, the difference of the drying condition can be reduced between the droplet arranged previously and the droplet arranged subsequently.
In an embodiment of the present invention, the solvent vapor is preferably removed by blowing a gas to the substrate surface. This method can effectively remove the solvent vapor even when the solvent of the liquid to be discharged is a solvent that has a large density and is likely to stay inside the substrate surface. The gas used for this method must be the one that does not react with the liquid to be discharged. Preferably, an inert gas, such as an argon gas or a nitrogen gas, is used.
In the embodiment of the method of the present invention described above, the gas is preferably blown always to the substrate surface on the rear side of the liquid discharge ports (on the rear side of the relative traveling direction of the liquid discharge port to the substrate) while the droplet is arranged at any position on the substrate.
When the substrate surface is divided into a plurality of belt-like regions in a certain direction, for example, and when the droplets are arranged while the liquid discharge ports are being moved relatively in the same direction in each of the regions, the method described above can compulsively remove the solvent vapor from the droplets of the regions, where the arrangement of the droplets are now being arranged, and can prevent the solvent vapor from flowing to the regions where the droplets are to be arranged subsequently and to the regions where the droplets have already been arranged. As a result, the drying condition of the droplet at each position inside the substrate surface can be rendered uniform.
The present invention further provides an ink jet apparatus including gas blowing means for blowing a gas to a surface of a liquid discharged surface on which droplets have already been arranged. This ink jet apparatus can easily perform the methods of the present invention.
In an ink jet apparatus according to one embodiment of the present invention, a gas blowing device has a tubular gas blowing part having a plurality of gas blowing holes formed in a longitudinal direction. It is preferable that this tube be fixed to an ink jet head having a liquid discharge port and its fixing position is on the rear side in the relative traveling direction to the surface of the ink jet to which the liquid is to be discharged.
In an embodiment of the ink jet apparatus according to the present invention, a gas blowing part is disposed in the proximity of the liquid discharge port, and is preferably constructed in such a fashion as to blow the gas at an angle of 30xc2x0 to 60xc2x0 to a perpendicular direction on the rear side of a relative traveling direction of the ink jet head to the surface of the liquid jet surface.
In one embodiment of the ink jet apparatus according to the present invention, the length of the gas blowing region in the longitudinal direction of the tube is preferably at least twice the size of the liquid jet surface in the longitudinal direction of the tube, and an ink jet head is preferably disposed at the center of the gas blowing region of the tube in its longitudinal direction.
In one embodiment of the ink jet apparatus according to the present invention, the tube is fixed also on the front side in the relative traveling direction of the ink jet head to the liquid discharged surface.
The present invention further provides a production method of an organic EL device for forming a thin film constituting an organic EL device by using the thin film formation method of the ink jet method according to the present invention, or by using the ink jet apparatus according to the present invention, and an organic EL device produced by this method.